1. Field of the Invention
The present invention relates to a plasma processing method and apparatus, and more particularly to an apparatus and method suitable for a continuous plasma processing system in which a substrate to be processed is intermittently and successively fed and subjected to plasma-processing when the substrate is in a stationary position.
2. Description of Prior Art
A plasma processing apparatus is widely used in the industry, in which materials are continuously processed in a plasma atmosphere by feeding the materials to be processed with holders for holding the materials thereon to a reactor, effecting desired modification or etching on the materials in the plasma atmosphere and taking out the processed materials. As examples of the plasma processing apparatuses, there may be mentioned a plasma etching apparatus of a sheet-feed type used in a preprecessing far semiconductor production, a sputter deposition apparatus for forming multi-layer films on a magnetic disk or an optical disk as disclosed in National Publication of Translated Version No. 62-502846 (PCT), etc.
Also well known is an etching method or an ion-assist film forming method by a high-energy ion utilizing a self-bias effect obtained in a high-frequency plasma environment by a difference in areas of electrodes, such as a reactive ion etching method, a high-hard carbon film forming method, etc. Although the processing is generally effected on the side of an electrode to which a high-frequency voltage is applied, the processing may also be effected on the side of a grounded substrate by increasing the area of the hot electrode to which the high-frequency voltage is applied, as disclosed in Japanese Patent Application Publication (KOKAI) No. 62-274080. The Application discloses a high-frequency electrode configured to surround plasma generated. The high-frequency electrode has, at a side thereof, an opening. A grounded substrate to be processed with a holder for holding the same is placed so as to cover the opening. Plasma is generated between the electrode and the substrate or the holder. With this arrangement, the area of a portion of the high-frequency electrode where it contacts the plasma generated can be much larger than that of a portion of the grounded side which contact the plasma. Thus, the film forming or etching can be effected on the grounded substrate by utilizing the self-bias effect. This is especially advantageous where a number of substrates are to be continuously processed, because the processing can be made on the substrate at the ground potential.
Conventional plasma processing apparatuses of a continuous processing type have such a disadvantage that when a drive mechanism or a substrate holder located around the substrate being processed is subject to film forming or heating when the plasma generated leaks. Particularly in a film forming process such as plasma CVD, films are liable to accumulate on the undesired parts. The so accumulated films will finally come off and result in foreign matters which will possibly adhere to the substrate.
When it is required to carry out another process such as heating, cooling, deposition, sputtering, CDV or the like in an area adjoining the plasma processing area, leakage of plasma causes various undesirable effects such as pollution of a heater in the processing area, causing trouble of the heater, or contamination of the film formed with foreign matters.
Another serious problem in the plasma processing method where self-bias effect is applied is that if the plasma spreads out of the processing area, a potential of the plasma becomes closer to the ground potential because it is difficult to obtain a high ratio of the area of the high-frequency electrode contacting the plasma to the area of the other part. This prevents desired processing on the substrate.
The cause of the problem lines in that the generated plasma leaks and expands if a clearance between the electrode and the substrate is large. The leakage of the plasma cannot be prevented by directly connecting the electrode to the substrate, because a difference in potential between the electrode and the substrate is as large as 100 V to 2,000 or 3,000 V. On the other hand, the clearance must be large enough to pass the substrate with the holder therethrough to transport the substrate into or from the area before and after the processing.
It is well known that the leakage of plasma can be prevented by decreasing the clearance between the electrode of a high voltage and the part of a ground potential. A gas pressure in the processing area is, however, from 0.01 to 9.5 Torr when conducting a plasma processing. To prevent the leakage of plasma at the pressure, the clearance must be from 1 to 3 mm. it is, however, difficult to pass the substrate through that narrow clearance, it is more difficult or impossible when the substrates have raised portions or have dimensional errors.